The core of a nuclear reactor typically comprises of a plurality of prism shaped nuclear fuel assemblies, supported on a core support plate. Placed above the assemblies is an upper core plate (UCP) designed for, among other things, locking into position the top nozzles of the nuclear fuel assemblies.
Each of the top nozzles of the nuclear fuel assemblies typically includes two holes, called “S shaped hole”, intended for cooperating with the centering pins of the upper core plate. The centering pins protrude under the upper core plate and is each engaged in an S shaped hole.
It is important for the nuclear fuel assemblies to be positioned properly inside the nuclear reactor core.
Indeed, during core loading operations, the nuclear fuel assemblies are first set in place inside the reactor core, and then the upper core plate and other reactor internals are replaced. During the setting in place of the upper core plate (UCP), the pins are engaged in the S shaped holes. If the pins and the S shaped holes of some assemblies are in an offset position relative to each other, the pins of the UCP can be entered with force in some S shaped holes.
This does not in any way interfere with the operation of the reactor, but during the subsequent shut down of the reactor, when the UCP is extracted out of the core, there is a risk of the nuclear fuel assemblies remaining attached to the upper plate of the core during the lifting of the UCP.
Such a situation is particularly problematic.